1. Field of the Invention
The present invention relates to the treating of mineral particles with insolubilizable compounds by an improved process to give paper with improved properties. It particularly relates to the use of calcium carbonate. One of the most important technical results of the invention is that there is reduced chemical reactivity of the calcium carbonate in the presence of papermaker's alum. It becomes an economic possibility to manufacture paper filled with calcium carbonate and sized by the traditional rosin-alum sizing process whilst avoiding difficulties which have in the past made such manufacture impracticable.
The present invention provides mineral fillers for the manufacture of sized paper which has an improved resistance to dusting at high filler loadings and improved burst strength and tensile strength over papers made with conventional fillers. In addition there is an increased rate of ink drying.
By use of the fillers of this invention advantages occur in the manufacturing process. The rate of drainage from the wire of the paper machine is improved and therefore the speed of the paper machine can be increased by up to 30% to produce the same quality of paper. Retention of the filler on the paper machine wire is increased and severe frothing, an almost insuperable difficulty heretofore encountered with rosin alum sizing and calcium carbonate filler, is controlled.
The use of calcium carbonate fillers is advantageous as loading or filler in paper because of improved durability. With natural chalk whiting there is a further advantage in economy over the commonly used material china clay. This fundamentally derives from the fact that practically all of the rock from a chalk quarry is converted to filler, whereas from a clay pit a large proportion of impurity, such as silica, is discarded. A further advantage in England of the chalk whitings as fillers, as compared with china clay, lies in the abundant supply and the convenient location of whiting manufacturing works.
2. Discussion of the Prior Art
The development of the use of chalk whiting as a paper filler has been considerably inhibited by difficulties which arise in the use of the traditional rosin soap/papermakers' alum sizing process in the conventional paper making processes. Although alternative sizing processes, such as the use of alkyl ketene dimer ("Aquapel", Trade Name of Hercules Powder Co.), cationic polymer emulsions ("Basoplast", Trade Name of B.A.S.F. (U.K.) Ltd), and solubilized carboxylated polymer ("Scripset", Trade Name of Monsanto Chemicals Ltd.), as sizing agents these each have individual disadvantages, and the large bulk of paper is still made by the traditional, rosin-alum sizing process. This process normally operates under acid conditions which are impossible to maintain in the presence of suspended calcium carbonate particles.
In a paper making process using soft water which is not recirculated (an open backwater system) sizing of cellulose pulp containing a calcium carbonate filler using rosin and papermakers' alum occurs readily producing a paper with physical properties very comparable to a paper made under similar conditions with a clay filler only.
This process is however generally impracticable; some mills run on hard water, most in order to save water use a recirculating system and employ backwater in the primary pulper and in the first make-up water of the machine chest.
It has been shown that under some mill conditions good sizing, when calcium carbonate is used as a filler, can be obtained by the use of alum and a high-free rosin emulsion size such as Bewoid, as marketed by Butler Malros Ltd. In the operation of this process, however, difficulties are experienced especially when a high proportion of the machine water is recirculated. With calcium carbonate as filler the pH is automatically buffered back to about 7.0 regardless of the amount of alum added so that this method can no longer be employed. In any case there is a tendency for papermakers to add an excess of alum to maintain the accustomed pH values. This leads to further difficulties.
During a paper making run there is a build-up of calcium sulphate in the circulating water of the paper machine due to reaction between the acidic alum and the calcium carbonate. This had disadvantages; the presence of sulphate ions is undesirable since sulphate tends to destroy sizing at neutral or slightly acidic pH values. Also the calcium ions are liable to react with the small amount of sodium rosinate which is present in the Bewoid size. The calcium rosinate which is thus formed has little or no sizing property and is hydrophobic and tends to froth to the surface of the web as it is forming on the paper machine stabilising air-bells and giving rise to bubble marks, "dandy marks" or "worm marking".
The literature contains many disclosures of processes which claim success in rosin alum sizing paper in the presence of acid reactive fillers but none of these processes have been industrially successful. On the other hand the present invention shows considerable cost savings because of the successful use of calcium carbonate as a filler.
`Pulp and Paper: Chemistry and Chemical Technology`, second edition, J. P. Casey; Interscience 1960, Volume II, p 987, states "Calcium carbonate would be an excellent filler except that it must be under alkaline conditions. Most paper is made at a pH of 4.5 - 5.5 and under these conditions, calcium carbonate reacts with the alum. This produces foam, changes the shade of coloured papers, increases pitch formation, and most serious of all reduces or completely destroys the sizing. Starch, especially potato, helps to control foam".
The above factors have effectively prevented calcium carbonate being used as a paper loading. Varieties of this filler are used as paper coating components, and so some finds its way via the broke back into the paper stock. To maintain the required acid conditions the calcium carbonate must be as far as possible washed out of the broke and/or completely dissolved in acid, so that acid papermaking conditions can be maintained.
The only escape from these difficulties would seem to be to treat the calcium carbonate filler so as to reduce the rate of reaction with acid in order that the required acidic conditions can be maintained on the paper machine wire without excessive use of alum. The present invention involves the treatment of acid reactive mineral particles before employing them in the final stages of the papermaking process. This novel process, when employed with suitable hydrophilic organic polymers confers many beneficial effects, detailed above, on the paper produced.
The process of the present invention for the first time enables calcium carbonate filler to be used with rosin-alum sizing without the above difficulties by reducing the rate of reaction between calcium carbonate and acidic alum, so enabling the pH to be maintained at a sufficiently low and controllable level for successful mill operation.
This process involves the formation of filler plus polymer agglomerates. The precise nature of these is uncertain but their preparation is not disclosed in the prior art.
The prior art details many processes for treatment of pigments and fillers mostly for use in rubbers, plastics and non-aqueous paints but in general such treatment is quite inappropriate for aqueous papermaking systems because of the hydrophobic or organophilic nature of the resulting pigments or fillers.
In papermaking processes these treated pigments and fillers do not wet with water and they accentuate the froth problems mentioned above. In addition they are inappropriate for use with calcium carbonate fillers during paper manufacture because they have little bonding action with cellulose fibres. Mostly they would tend to destroy bonding between hydrophilic filler and hydrophilic fibres and between fibres. To gain advantages in strength, hydrophilic hydroxylated polymers notably gel-forming polysaccharides and polyalcohols, which are not dissimilar in character to the cellulose itself, certain carboxylated polymers, or proteins can be used. Presence of a polymer not of the above type such as an alkali-solubilised acrylic emulsion may be advantageous to help insolubilization or to modify rheology for forming filiform filler polymer agglomerates.
It is customary in the papermaking art to add starch or other polymers of a hydrophilic nature to the paper machine at the beater or elsewhere. These have a fibre-bonding but not specifically a filler bonding action. Only by choosing polymers and polymer mixtures which have a similar hydrophilic nature to the papermaking fibres and forming the filler particle agglomerates which are characteristic of the present invention before individual filler particles can interact with the fibres can the process of the invention be made to succeed.
As stated above the materials inappropriate for papermaking, soaps and hydrophobic resins would, on the paper machine cause pitch and frothing difficulties. Such materials tend to stick to wire, rolls, and felts under the conditions of sizing of clay-loaded paper. One trouble experienced with fatty acid soaps is wire blinding, particularly severe with hard water. As stated by Casey (p. 1097) this becomes worse in the presence of calcium carbonate. Also such alkali-solubilised soaps adsorb onto the calcium carbonate particles, making them hydrophobic. De-wetting and froth-spume difficulties arise. The result is that good paper cannot be made because of foaming.
The prior art also discloses treatment of fillers and pigments with silicates. This does not appear to be effective in the process of the present invention. Silicates are not organic hydrophilic polymers so that this does not constitute prior disclosure. Examples of prior art but not prior disclosure of this invention are:
U.S. Pat. No. 2,259,483 describes the treatment of a pigment with an alkali metal silicate and a water-soluble salt capable of precipitating the silicate in an insoluble form to form a hydrophobic coating.
U.S. Pat. No. 2,296,639 describes the treatment of a pigment with a soluble silicate and precipitation of the silicate with alum.
U.S. Pat. No. 3,025,179 describes the treatment of a pigment with hydrophobic organic compound and alkali metal salts of higher fatty acids, and mono or disubstituted polysiloxanes. The coating is precipitated and the water removed.
U.S. Pat. No. 3,642,510 describes the formation of hydrophobic titanium oxide pigment by adding an alkali metal salt of a carboxyl compound to a slurry of titanium oxide and precipitation of the material.
U.S. Pat. No. 3,348,959 describes the treatment of pigments with hydrophobic coatings formed from alkali and ammonium salts of various carboxylic acids with boron, antimony, arsenic, zinc lead or chromium organic salts.
The processes disclosed in these patents are inappropriate for papermaking because of the hydrophobic nature of the filler produced.
It is also known in the art, U.S. Pat. No. 2,140,394 - Ruff, to coat mineral particles with starch by heating the mineral particles with starch particles so that the starch particles burst surrounding the particles with colloidal starch. However, this process needs a high proportion of starch to filler and the temperature of all the filler and water in which it is suspended to be raised. This requires a large amount of heat if it is to be used successfully in a paper mill, making the process uneconomic and not industrially useful.
It is evident that U.S. Pat. No. 2,140,394 - Ruff does not disclose the addition of a gellant, flocculant or precipitant to the starch. The gelling process is only carried out by the method of cooling. There is little doubt in this matter that the method disclosed by Ruff would be effective; the only difficulty is that it is not economic. It is not until the present invention that an economic process of making paper has been proposed in which calcium carbonate is used as a filler in conjunction with conventional rosin and alum sizing without the associated problems of acid reaction between the alum and calcium carbonate, frothing and many other problems set out above.
U.S. Pat. No. 3,257,267 (Hay) is concerned with the incorporation of certain materials into fibrous materials by embedding the material to be incorporated in a gel matrix prior to addition to the fibres. The production of gels is described and the application of the method to filler particles in papermaking. It is, however, clear that this disclosure is concerned solely with physical or mechanical means and not chemical means to produce "particulate masses". There is no suggestion therein of the formation of stable agglomerates by the use of chemical insolubilization agents. At least one of the gel systems produced by the Hay process would swell in time and break up, such system being polyvinyl alcohol (PVA)/borax. For this reason, and as will be explained hereinafter, the use of PVA/borax to produce stable agglomerates is not possible in the present invention. Data will be presented hereinafter indicating the large differences between the Hay process and the present invention. Suffice it to say at this point that papers produced by the present invention possess desirable permanence properties and have very satisfactory wet web and overall strength properties.